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Fix race condition when two processes create a hard link to a file in .links

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This is a problem because one process may set the immutable bit before
the second process has created its link.

Addressed random Hydra failures such as:

error: cannot rename `/nix/store/.tmp-link-17397-1804289383' to
`/nix/store/rsvzm574rlfip3830ac7kmaa028bzl6h-nixos-0.1pre-git/upstart-interface-version':
Operation not permitted
This commit is contained in:
Eelco Dolstra 2012-08-05 21:41:44 -04:00
parent 108e14bb18
commit b6c989b801
1 changed files with 37 additions and 27 deletions
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44
src/libstore/optimise-store.cc
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@ -123,9 +123,6 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % linkPath);
Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
% nixStore % getpid() % rand()).str();
/* Make the containing directory writable, but only if it's not
the store itself (we don't want or need to mess with its
permissions). */
@ -140,40 +137,53 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
so make it mutable first (and make it immutable again when
we're done). We also have to make ‘path’ mutable, otherwise
rename() will fail to delete it. */
makeMutable(linkPath);
MakeImmutable mk1(linkPath);
makeMutable(path);
MakeImmutable mk2(path);
/* Another process might be doing the same thing (creating a new
link to ‘linkPath’) and make ‘linkPath’ immutable before we're
done. In that case, just retry. */
unsigned int retries = 1024;
while (--retries > 0) {
makeMutable(linkPath);
MakeImmutable mk1(linkPath);
Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
% nixStore % getpid() % rand()).str();
if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
if (errno == EMLINK) {
/* Too many links to the same file (>= 32000 on most file
systems). This is likely to happen with empty files.
Just shrug and ignore. */
/* Too many links to the same file (>= 32000 on most
file systems). This is likely to happen with empty
files. Just shrug and ignore. */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
return;
}
if (errno == EPERM) continue;
throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
}
/* Atomically replace the old file with the new hard link. */
if (rename(tempLink.c_str(), path.c_str()) == -1) {
if (errno == EMLINK) {
/* Some filesystems generate too many links on the rename,
rather than on the original link. (Probably it
temporarily increases the st_nlink field before
decreasing it again.) */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
/* Unlink the temp link. */
if (unlink(tempLink.c_str()) == -1)
printMsg(lvlError, format("unable to unlink `%1%'") % tempLink);
if (errno == EMLINK) {
/* Some filesystems generate too many links on the
rename, rather than on the original link.
(Probably it temporarily increases the st_nlink
field before decreasing it again.) */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
return;
}
if (errno == EPERM) continue;
throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
}
break;
}
if (retries == 0) throw Error(format("cannot link `%1%' to `%2%'") % path % linkPath);
stats.filesLinked++;
stats.bytesFreed += st.st_size;
stats.blocksFreed += st.st_blocks;